Process for the preparation of naphthalene-1,4-dicarboxylic acid

It is already known that naphthalene-1,4-dicarboxylic acid can be prepared by hydrolysis of 1,4-dicyanonaphthalene [compare R. Scholl and H. Neumann, Ber. 55 (1922), 121 and E. F. Bradbrook and R. P. Linstead, J. Chem. Soc. (London) 1936, 1739-1744]. The processes described in these publications for the preparation of the dinitrile use suitable naphthalenesulfonic acids as starting materials, which are converted into 1,4-dicyanonaphthalene in a potassium cyanide melt. However, these processes require extreme conditions and lead to a relatively large amount of dark-colored by-products, and are therefore not particularly suitable for industrial production of naphthalene-1,4-dicarboxylic acid.

It has now been found that naphthalene-1,4-dicarboxylic acid can be prepared in a high purity and in a manner which can easily be carried out on an industrial scale by reacting 1,4-dihalogenonaphthalene with at least 2 moles of copper-I cyanide per mole of dihalogenonaphthalene in polar aprotic solvents, if appropriate in the presence of catalytic amounts of potassium iodide and copper sulfate, and subjecting the copper salt complex of 1,4-dicyanonaphthalene thus obtained to alkaline hydrolysis.

Suitable dihalogenonaphthalenes are 1,4-dichloronaphthalene, which has already been described in the literature a long time ago (compare German Reichspatent No. 286,489), and 1,4-dibromonaphthalene, which has also already been known for a long time (compare J. S. Salkind and S. B. Faerman, Journal of the Russian Physical Society, Chemical Section 62 (1930), 1021-1032). Both the dihalogenonaphthalenes can be prepared in a simple manner from the parent substance naphthalene, which is readily accessible on an industrial scale. The amount of copper-I cyanide used varies between 2 and 2.5 moles per mole of the dihalogeno compound. The polar aprotic solvent used is N-methylacetamide, dimethylacetamide, dimethylsulfoxide, N-methylpyrrolidone, hexamethylphosphoric acid triamide, quinaldine or, preferably, dimethylformamide or quinoline. In the case of the dibromo compound, dimethylformamide can advantageously be used. The dichloro compound, which reacts more slowly, is preferably reacted in the higher-boiling solvent quinoline. The reaction is carried out at temperatures from 150° to 250° C., advantageously at the boiling point of the solvent employed. It is also advantageous to accelerate the reaction by adding a catalyst. A mixture of potassium iodide and copper sulfate can be used for this purpose.

In this reaction, 1,4-dicyanonaphthalene is obtained as a copper-I salt complex having the following composition: ##STR1## and can be isolated after the reaction mixture has been cooled and after water or a low-boiling solvent has been added. Lower alcohols, preferably methanol or ethanol, are used as the low-boiling polar solvent.

The still moist copper-salt complex is then directly subjected to alkaline hydrolysis with a strong alkali, for example with excess 10-35% strength sodium hydroxide solution, at the boiling point by known methods, without the 1,4-dicyano-naphthalene being liberated. The pH value of the solution is then adjusted to 7 to 8 and the copper oxide which thereby precipitates quantitatively is separated off. The naphthalenedicarboxylic acid is separated out of the filtrate by further addition of mineral acid up to a pH value of 2, and is separated off.

Working up can also be carried by a procedure in which isolation of the copper salt complex of 1,4-dicyanonaphthalene is dispensed with and, when the replacement of cyanide by halogen has ended, the reaction mixture is hydrolyzed directly with a strong alkali, such as, for example, 10-35% strength sodium hydroxide solution.

If quinoline or quinaldine is used as the solvent, this can be distilled off with steam during the hydrolysis.

The process according to the invention can also be carried out by using, as the starting substances, 1,4-dihalogenonaphthalenes which are still contaminated with other dihalogenonaphthalenes which are position isomers, and subjecting the resulting crude naphthalene-1,4-dicarboxylic acid to purification. Possible dihalogenonaphthalenes which are position isomers and which can be contained in the 1,4-dihalogenonaphthalene are, above all, the 1,5- and 2,6- as well as the 2,7-isomers. For purification, the crude naphthalene-1,4-dicarboxylic acid is dissolved in a suitable solvent, after being dried, and the sparingly soluble isomers, that is to say naphthalene-1,5- and/or -2,6- and/or -2,7-dicarboxylic acid are separated off by filtration. Low-boiling glycol monoethers, such as ethylene glycol monomethyl ether and ethylene glycol monoethyl ether, are chiefly suitable for the purification operation.

The naphthalene-1,4-dicarboxylic acid obtained by the process according to the invention is used as a precursor for dyestuffs, UV absorbers, scintillators and optical brighteners. It is free from colored impurities and can be employed directly for further synthesis.

The following examples illustrate the invention in more detail. The temperatures are given in °C.